a) Field of the invention
The present invention relates to an injection carburetor for internal combustion engines, and more particularly to a fuel supply system provided in a suction tube which can meter a flow rate of fuel to render an airfuel ratio of a gas mixture constant by balancing a difference between the negative pressure produced in the suction tube and the atmospheric pressure with a difference in fuel pressure between the upstream side and the downstream side of an orifice provided in a fuel passage.
b) Description of the prior art
In the past, a system metering a flow rate of fuel in accordance with relationship between the flow rate of fuel passing through an orifice and a difference in fuel pressure between the upstream side and the downstream side of the orifice, as in fuel injection systems of stationary venturi type carburetors and U.S. Pat. application Ser. NO. 341,827, has been designed so that only the fuel supplied to an engine passes through the orifice. When the passed fuel is metered by the orifice, as diagrammed in FIG. 1, the fuel pressure difference is proportional to the square of the fuel flow rate, with the result that, for example, if the fuel of the amount six times the minimum supply fuel flow rate of the system flows through the orifice, the fuel pressure difference will be increased as much as 36 times the difference at that time and reach a limit value in practical use. However, general engines for automobiles, which need to be capable of metering the fuel supply flow rate from the minimum to about 40 times that, cannot make use of such a conventional fuel injection system as in the foregoing. Accordingly, in order to solve this problem, as in U.S. Pat. application Ser. No. 352,299, a system has been proposed in the past which is constructed to arrange at least two fuel control units for a slow zone and a main zone. This system, however, has defects that its structure is complicated and the transition from the slow zone to the main zone is not performed smoothly. Further, although another system is available which is capable of covering such a wide metering range as is mentioned above in the fuel supply system with a single fuel control unit, like SU carburetors, this system brings about defects that since the arrangement is such that the fuel flow rate is metered by change of the sectional area of the fuel passage (i.e., change of channel resistance) according to the flow rate of air, metering accuracy is reduced.